This proposal would employ a novel technique for transplanting trophoblast to ectopic sites outside the uterus to investigate the mechanisms and consequences of recognition of trophoblast Major Histocompatibility Complex (MHC) class I antigens by maternal T lymphocytes. The principal hypothesis underlying this research is that trophoblast cells have the capacity to induce a state of 'split immunological tolerance' in the pregnant female leading to activation of the B cell compartment, with simultaneous regulation of T cell-mediated immunity. Three specific aims address complementary aspects of this question. In Aim 1 the trophoblast transplant system would be used to determine if ectopic trophoblast placed adjacent to conventional grafts can prolong their survival, or conversely if conventional grafts can trigger the destruction of near-by trophoblast. These experiments would distinguish between active immune suppression and 'immunological ignorance' of the transplants. Aim 2 would focus on events in the pregnant uterus modeled by the trophoblast transplants. Experiments would compare the expression of interferon gamma and Interleukin-10 (IL-10) in the endometrial cups of horse and mule pregnancy to determine whether interspecies pregnancy fails to generate the T cell tolerance characteristic of normal pregnancy. Lymphocytes infiltrating endometrial cups would be recovered from biopsies and characterized for phenotype and immune function in vitro. The expression of the Foxp3 transcription factor and the IL-2 receptor, as markers for regulatory T cells, would be assayed in these cells. Using co-cultures of trophoblast, lymphocytes, and antigen presenting cells, the experiments of Aim 3 would test the hypothesis that trophoblast cells can act directly in antigen presentation to induce tolerogenic or productive immune responses to MHC class I antigens. These experiments hold the promise of identifying novel mechanisms by which the trophoblast cells protect themselves from destructive T cell mediated immune responses. These mechanisms have potential application in human pregnancy failure and in related fields, including clinical organ transplantation and tumor immunology.